A method of taking out a part of exhaust gases from an exhaust system to return the same to an intake system of an engine again to add the same to a mixture is called EGR (Exhaust Gas Recirculation). EGR is thought to be a method that is effective in purification of exhaust gases of an engine and enhancement in thermal efficiency because it produces much effect such as suppression of generation of NOx (nitrogen oxide) reduction in pumping loss, that reduction in radiation loss to a cooling liquid, which is accompanied by temperature drop of combustion gases, an increase in specific heat ratio, which is caused by changes in quantity and composition of working gas, and enhancement in cycle efficiency accompanying such increase.
However, when an EGR gas is increased in temperature and in quantity, an EGR valve is degraded in durability due to a thermal influence thereof and suffers in some cases from early breakage, and it is recognized that there is a need for a water-cooled construction for the purpose of prevention such degradation and breakage and fuel consumption is decreased due to that reduction in charging efficiency, which is caused by an increase in intake-air temperature. In order to avoid such situation, there is used a device that cools an EGR gas with a cooling liquid of an engine, a cooling medium for car air conditioners, a cooling wind, or the like.
Various coolers of heat exchanger type have been proposed as a conventional cooling device for an EGR gas.
There are, for example, a double tube type heat exchanger, in which an outer tube for having a liquid passing therethrough is arranged outside an inner tube for having a gas passing therethrough, heat exchange is performed between the gas and the liquid, and metallic corrugated sheets are inserted as fins into the inner tube (see JP-A-11-23181), a double tube type heat exchanger, in which an inner tube and an outer tube are provided, and either of a high-temperature side fluid passage and a low-temperature side fluid passage is provided on a side of the inner tube and a side of the outer tube, respectively (see JP-A-2002-350071), a double tube type heat exchanger comprising an inner tube for having a medium being cooled, flowing inside, an outer tube provided in a manner to surround an outer periphery of the inner tube with a gap therebetween, and radiating fins having a thermal stress relaxing function and arranged inside the inner tube (see JP-A-2000-111277), a double tube type heat exchanger comprising an inner tube for having a medium being cooled, flowing inside, an outer tube provided in a manner to surround an outer periphery of the inner tube with a gap therebetween, and cross fins arranged inside the inner tube (see JP-A-2003-21478), an EGR gas cooling device, in which a cooling pipe (heat transfer pipe) is brought into contact with and spirally wound around an outer peripheral surface of an EGR gas pipe (see JP-A-9-88730), and an EGR gas cooling device constructed such that a cooling pipe (heat transfer pipe) extends through an outer peripheral wall of an EGR gas pipe to be inserted into the EGR gas pipe (see JP-A-9-88731).